The use of affinity tags that are genetically coded and linked to a target protein to facilitate its purification is well established in the research community. Examples of commonly used such tags are glutathione-S-transferase (GST), hexa-histidine and maltose binding protein. In order to purify the affinity tagged protein, chromatography media with an immobilized specific ligand for the affinity tag is used. The chromatography media used should exhibit a high binding capacity for the target protein and yield highly pure target protein at high recoveries in a short time frame (“quality criteria”). Chromatography media used for this purpose is often in the form of beads but can also be monolithic or in other formats. Magnetic chromatography media is useful for many applications and has a number of benefits related to the purification of affinity tagged proteins. One benefit is that the magnetic bead format, in contrast to commonly used column formats, works well with samples that contain small particles like cell debris. Such small particles are often present in cell lysates which is a common start material for purification of tagged proteins. Another benefit is that purification of tagged proteins with magnetic chromatography media can be performed rapidly with very simple equipment, and with little effort. Yet another benefit is that protocols using magnetic chromatography media readily can be automated.
Magnetic chromatography media can be produced by immobilization of the desired ligand directly on an iron-containing mineral, but it is preferred to have the iron-containing mineral coated with e.g., a polymer that is inert and allows for control of the ligand density obtained during the coupling reaction. Commercially available products like His Mag Sepharose™ Ni and Streptavidin Mag Sepharose (GE Healthcare Biosciences AB) uses highly cross-linked agarose for the coating to which the ligand is attached. Such highly cross-linked agarose is also used in conventional (i.e., non-magnetic) chromatography media. For unknown reasons, magnetic chromatography media with highly cross-linked agarose and immobilized glutathione (for purification of GST-tagged proteins) does not fulfill all the quality criteria outline above. More specifically, such media exhibits a low binding capacity for GST-tagged target proteins.